1. Field of the Invention
The present invention relates to a solid-state image sensor and a method of driving the same, and more particularly to an amplifying type solid-state imaging device such as a MOS image sensor where unit pixels each having an amplifying function are arrayed in the form of a matrix of rows and columns, and also to a method of driving such an image sensor.
2. Description of the Related Art
FIG. 13 shows an exemplary structure of a conventional amplifying type solid-state image sensor known heretofore in the related art. In this diagram, a unit pixel 106 is composed of a photodiode 101, an FD (floating diffusion) read MOS transistor 102, an FD amp MOS transistor 103, an FD reset MOS transistor 104, and a vertical select MOS transistor 105. In this structure, a gate electrode of the FD read MOS transistor 102 is connected to a vertical read line 107, a gate electrode of the FD reset MOS transistor 104 to a vertical reset line 108, a gate electrode of the vertical select MOS transistor 105 to a vertical select line 109, and a source electrode of the vertical select MOS transistor 105 to a vertical signal line 110, respectively.
A horizontal select MOS transistor 112 is connected between one end of the vertical signal line 110 and a horizontal signal line 111. The operation of each pixel is controlled per row by three kinds of vertical scanning pulses xcfx86VSn, xcfx86VTn, and xcfx86VRn, outputted from a row-select vertical scanning circuit 113, and a pixel signal is outputted to the horizontal signal line 111 via the horizontal select MOS transistor 112 which is controlled by a horizontal scanning pulse xcfx86Hm outputted from a column-select horizontal scanning circuit 114. At this time, the signal charge stored in the photodiode 101 through photoelectric conversion is converted into a signal current by the FD amp MOS transistor 103 and then is delivered as an output signal of the image sensor.
However, in the known amplifying type solid-state image sensor of the above structure, there exists a problem of characteristic deviation in the active elements constituting each pixel, principally in the FD amp MOS transistor 103, and particularly relative to deviation of the threshold voltage Vth of the MOS transistor. And such deviation is included directly in the output signal of the image sensor. Since this characteristic deviation has a fixed value per pixel, it appears as a fixed pattern noise (FPN) in the picture displayed on a screen. For suppressing such fixed pattern noise, it is necessary to externally connect to the device a noise elimination circuit using a frame memory or a line memory, so as to eliminate any noise component derived from the characteristic deviation in the pixel. As a result, the scale of the camera system is rendered larger correspondingly to the noise elimination circuit connected thereto externally.
In comparison with the above, there is contrived another amplifying type solid-state image sensor which has a structure of FIG. 14 and is capable of internally suppressing such fixed pattern noise in the device. The difference of this solid-state image sensor resides in the point that, although its unit pixel 106 is structurally the same as FIG. 13, a horizontal output circuit 115 is provided for suppressing the fixed pattern noise derived from the characteristic deviation in the pixel 106, and this horizontal output circuit 115 executes a process of taking the difference between pre-read and post-read (pre-reset and post-reset) signals of the pixel 106.
In FIG. 14, a load MOS transistor 116 serving as a load to the source follower operation of an FD amp MOS transistor 103 is connected between a vertical signal line 110 and the ground. Further, one main electrode of each of paired signal switch MOS transistors 117 and 117xe2x80x2 is connected to the vertical signal line 110. And a pair of signal holding capacitors 118 and 118xe2x80x2 are connected respectively between the ground and the other main electrodes of such paired signal switch MOS transistors 117 and 117xe2x80x2.
Further a pair of horizontal select MOS transistors 112 and 112xe2x80x2 are connected respectively between the other main electrodes of the paired signal switch MOS transistors 117, 117xe2x80x2 and a pair of horizontal signal lines 111, 111xe2x80x2. And a noninverting (+) input terminal and an inverting (xe2x88x92) input terminal of a differential amplifier 119 are connected respectively to the pair of horizontal signal lines 111 and 111xe2x80x2.
In the amplifying type solid-state image sensor of the above structure, pixel pre-reset and post-reset signals are held respectively in signal holding capacitors 118, 118xe2x80x2 via the signal switch MOS transistors 117, 117xe2x80x2 and then are supplied to the differential amplifier 119 via the horizontal select MOS transistors 112, 112xe2x80x2 and the horizontal signal lines 111, 111xe2x80x2. Subsequently, the difference between the pixel pre-reset and post-reset signals is taken in the differential amplifier 119 to thereby eliminate the fixed pattern noise derived from the characteristic deviation in each unit pixel.
Although it is possible in the amplifying type solid-state image sensor of the above structure to suppress the fixed pattern noise derived from the characteristic deviation in each unit pixel, the pixel pre-reset and post-reset signals reach the differential amplifier 119 via separate signal paths, so that the characteristic deviations relative to the paired signal switch MOS transistors 117, 117xe2x80x2 and the paired horizontal select MOS transistors 112, 112xe2x80x2 appear in the picture as fixed pattern noises with vertically correlated streaks. Therefore, this structure also requires an external correction circuit for suppressing the fixed pattern noises with vertical streaks.
It is an object of the present invention to provide an improved amplifying type solid-state image sensor which is capable of suppressing, within the device, any fixed pattern noise derived from characteristic deviation in each unit pixel and also other fixed pattern noise of vertical streaks.
And another object of the invention is to provide a method of driving such an image sensor.
According to one aspect of this invention, there is provided a solid-state image sensor which comprises, in each of unit pixels arrayed to form a matrix of rows and columns, a photoelectric conversion element, an amplifying element having a storage to store a signal charge transferred thereto from the photoelectric conversion element and serving to convert the signal charge of the storage into an electric signal, and a selector switch for selectively outputting the pixel signal from the amplifying element to a vertical signal line. The image sensor further comprises, in each of the unit pixels, a reset circuit for resetting the storage of each amplifying element every time a pixel signal is outputted from each unit pixel.
According to another aspect of this invention, there is provided a method of driving a solid-state image sensor having the above structure. The method comprises the steps of resetting the storage of each amplifying element every time a pixel signal is outputted from each of the unit pixels; then delivering a pre-reset signal and a post-reset signal from each unit pixel and transferring such signals via a common transfer path; and taking the difference between the pre-reset signal and the post-reset signal.
In each of the unit pixels constituting the solid-state image sensor of the structure described above, the storage of each amplifying element is reset every time a pixel signal is outputted, so that a pre-reset signal and a post-reset signal per pixel are outputted successively from each of the unit pixels. In this case, fixed pattern noise derived from any characteristic deviation in the pixel is generated as an offset component from the amplifying element of each pixel. Therefore, the noise component can be canceled by taking the difference between the pre-reset signal and the post-reset signal. Since the pre-reset and post-reset signals are outputted from a vertical signal line to a horizontal signal line via a common signal path, fundamentally none of vertically correlated streak noise components is generated.
The above and other features and advantages of the present invention will become apparent from the following description which will be given with reference to the illustrative accompanying drawings.